It is well-known that the temperature of a resistance heating element, such as a soldering iron, can be controlled by controlling the voltage to the iron, for instance by putting resistance in series with it during idle periods so as to reduce the tendency of the soldering tip to overheat, burning the flux to contaminate the tip, and oxidizing the tip. The heating elements in most soldering iron tips have a great deal of thermal inertia so that it takes a considerable time to reheat the iron if it is permitted to cool below a temperature at which it will melt the solder. Therefore, most irons are permitted to remain turned ON at full voltage continuously, thereby causing rapid deterioration of the tip. For instance, U.S. Pat. Nos. 2,494,840 to Stephenson; 2,543,103 to Friesen and 2,550,748 to Woltz all show soldering iron stands having means thereon for switching a resistance in series with the soldering iron element during rest periods for the purpose of reducing the voltage to the iron and thereby reducing its tendency to overheat.
U.S. Pat. No. 2,969,449 to Tyler shows a soldering iron heat control system having a number of different selectable current paths by which the heat of the tip can be controlled.
U.S. Pat. No. 3,551,639 to Gotley shows an iron having a heat sensing element near the tip which is used to control the temperature of the iron in an automatic manner. U.S. Pat. No. 3,716,692 to Schick et al shows a soldering iron temperature control using an SCR for control purposes, and resembles somewhat the concept used in the afore-mentioned Gotley patent. U.S. Pat. No. 3,646,577 to Ernst also shows a temperature control system in the handle of the iron using an SCR type of device for controlling the temperature of the iron by turning the current to the iron alternately ON and OFF. This patent teaches the use of a voltage source which is greater than the rating of the heating element in the iron, the voltage source being turned ON and OFF in response to the temperature of the tip and being capable of rapid heating of the tip when the iron is turned ON.
U.S. Pat. No. 3,670,138 to Schmeige et al shows a welding system in which the current is controlled by time duration, but this circuit merely turns the current OFF after an interval without cycling the current ON and OFF in a continuously alternating sequence.